The most important implication of this result is that the AFM moments of the two slabs are not orthogonal unlike those in their individual systems [1]}, [2]}, [3]}, [4]}. To confirm this, we simulated the \(L\) -modulation of the (-0.5 0.5 \(L\) ) magnetic peaks [5]} because the relative change in the magnetic structural factor at different \(L\)  positions is primarily driven by the included angle of the AFM moments between the monolayer and bilayer slabs. We assumed the same moment sizes for both slabs and compared two simple scenarios: (i) AFM moments in \(ab\) -plane for monolayer and along \(c\) -axis for bilayer; (ii) AFM moments in \(ab\) -plane for both monolayer and bilayer with a parallel alignment. The former has a 90\(^{\circ }\)  included angle, while the latter's is zero. Comparing these two scenarios not only captures the impact of the included angle but also takes into account the in-plane moments in at least one slab according to the magnetization data. As can be seen in Fig.3(f), scenario (ii) clearly reassembles the observed modulation much better than (i). In particular, the intensity at \(L= 17.5\)  is strongest in (ii), whereas \(L= 16.5\)  and 18.5 are the two largest ones in (i), in sharp contrast to the experiment. Their distinct behaviors is due to the fact that the form factors of the two slabs are destructively superimposed at \(L= 16.5\)  and 18.5 and constructively added up at \(L= 17.5\) . Therefore, relative to the peak at \(L= 15.5\)  where the superposition is neither constructive nor destructive, having a strong peak at \(L= 17.5\)  and a weak peak at \(L= 16.5\)  is a signature that the two slabs have similar form factors and hence a small included angle. While including more variables and subtle modifications to the magnetic structure may further improve the agreement of the simulation with the experiment [5]}, this analysis demonstrates that the HSL cannot be regarded as a simple addition of the two subsystems.
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